1. Field of the Invention
The present invention relates to a stage for holding a wafer in a process gas atmosphere during, for example, a process of manufacturing a semiconductor device.
2. Description of the Related Art
Various processes, such as CVD (Chemical Vapor Deposition), PVD (Plasma Vapor Deposition) and etching, are performed in a process of manufacturing a semi-conductor device. Among the devices used in these processes are stages that hold a disc-shaped semiconductor wafer cut from an ingot. Each stage has a plate and a stem. The plate equips a heater or a high-frequency electrode. The stem supports the plate. The heater or high-frequency electrode is connected to a lead wire. The lead wire extends through the stem, which is formed a hollow cylinder.
The plate and the stem are exposed to a fluoric gas atmosphere and heated by the heater. In view of this, they are made of ceramics such as aluminum nitride. The plate (susceptor block) and the stem (support base) are secured to each other by screws. Refer to, for example, Jpn. Pat. Appln. KOKAI Publication 7-153706 (paragraphs 0032 to 0034, and FIG. 4). To prevent the lead line from being corroded by the process gas, such as fluoric gas, leaking out at the fitting portion between the plate and the stem, inert gas such as argon gas is introduced into the stem at a pressure higher than the pressure of the ambient gas.
Semiconductor wafer heaters are known which have a hollow cylindrical member (stem) and a ceramics heater (plate). Such a wafer heater is disclosed in Japanese Patent No. 3131010 (see paragraphs 0007, 0008 and 0014, and FIG. 1). The hollow cylindrical member has a flange at one end. The ceramics heater has a resistance heater embedded and terminals exposed at the back. The semiconductor wafer heater has an annular member provided between the flange and the back. The annular member is shaped like an O-ring and made of soft metal. A support member is hooked on the flange formed on the outer periphery of the ceramics heater. A holding plate is hanged on the hollow cylindrical member. Bolts fasten the support member and the holding plate together. They apply a load at the fitting portion between the hollow cylindrical member and the ceramics heater. The fitting portion is thereby sealed airtight. The hollow cylindrical member and the ceramics heater are discrete units. Thus, one of them can be separated from the other if cracks have developed in it and can be replaced by a new one. The soft metal, i.e., the material of the annular member, is platinum.
If the susceptor block and the support base are directly fastened with blots, however, the inert gas may leak from inside through the gap between the susceptor block and the support base. Further, the heat of the heater may be conducted through the fitting portion, causing a heat loss. A temperature difference may occur on the surface of the susceptor block. If a temperature difference occurs, the process conditions for manufacturing the semiconductor device becomes unstable. In some cases, the susceptor block may be broken due to a thermal stress.
If a platinum O-ring for annular member is inserted and maintains airtight between the ceramics heater and the hollow cylindrical member, the interior of the hollow cylindrical member will be exposed to the atmosphere. Therefore, heat may easily radiate, and the terminals or conductors may be readily oxidized. Since the ceramics hater is held by the support member at its outer periphery, it may be bent, particularly when its outer periphery is remote from the annular member. This decreases the flatness of the ceramics heater and impairs the sealing between the ceramics heater and the annular member. Since the ceramics heater and the hollow cylindrical member have different thermal expansion coefficients, the fitting portion between them may become less airtight than desired. Inevitably, a complex mechanism must be used to exert a stable force that pushes the ceramics heater and hollow cylindrical member to the annular member.